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Strahlentherapie und Onkologie

, Volume 194, Issue 1, pp 50–59 | Cite as

Air–electron stream interactions during magnetic resonance IGRT

Skin irradiation outside the treatment field during accelerated partial breast irradiation
  • Jong Min Park
  • Kyung Hwan ShinEmail author
  • Jung-in Kim
  • So-Yeon Park
  • Seung Hyuck Jeon
  • Noorie Choi
  • Jin Ho Kim
  • Hong-Gyun Wu
Original Article

Abstract

Purpose

To investigate and to prevent irradiation outside the treatment field caused by an electron stream in the air generated by the magnetic field during magnetic resonance image-guided accelerated partial breast irradiation (APBI).

Materials and methods

In all, 20 patients who received APBI with a magnetic resonance image-guided radiation therapy (MR-IGRT) system were prospectively studied. The prescription dose was 38.5 Gy in 10 fractions of 3.85 Gy and delivered with a tri-cobalt system (the ViewRay system). For each patient, primary plans were delivered for the first five fractions and modified plans with different gantry angles from those of the primary plan (in-treatment plans) were delivered for the remaining five fractions to reduce the skin dose. A 1 cm thick bolus was placed in front of the patient’s jaw, ipsilateral shoulder, and arm to shield them from the electron stream. Radiochromic EBT3 films were attached to the front (towards the breast) and back (towards the head) of the bolus during treatment. Correlations between the measured values and the tumor locations, treatment times, and tumor sizes were investigated.

Results

For a single fraction delivery, the average areas of the measured isodoses of 14% (0.54 Gy), 12% (0.46 Gy), and 10% (0.39 Gy) at the front of the boluses were as large as 3, 10.4, and 21.4 cm2, respectively, whereas no significant dose could be measured at the back of the boluses. Statistically significant but weak correlations were observed between the measured values and the treatment times.

Conclusion

During radiotherapy for breast cancer with an MR-IGRT system, the patient must be shielded from electron streams in the air generated by the interaction of the magnetic field with the beams of the three-cobalt treatment unit to avoid unwanted irradiation of the skin outside the treatment field.

Keywords

Magnetic field Magnetic resonance image-guided radiation therapy Out-of-field exposure Radiation protection Organs at risk 

Luft-Elektronenstrahl-Interaktionen in der Magnetresonanztomographie-geführten IGRT

Hautbestrahlung außerhalb des Zielgebiets während einer akzelerierten Teilbrustbestrahlung

Zusammenfassung

Zielsetzung

Beim Einsatz eines Magnetresonanztomographie(MRT)-geführten Bestrahlungsgeräts kann durch die Wechselwirkung von Magnetfeld und Strahlenquelle unerwünscht ein Elektronenstrahl außerhalb der Therapiefelder entstehen. In dieser Studie wurde für akzelerierte Teilbrustbestrahlungen (APBI) diese Strahlung abgeschirmt und näher untersucht.

Arbeitsmittel und Verfahren

Für diese Studie wurden prospektiv 20 Patientinnen ausgewählt, die eine APBI mit einer MRT-geführten Strahlentherapie (MR-IGRT) erhielten. Die Bestrahlung erfolgte mit einem Tri-Kobalt-System, das mit einem Magnetresonanztomographen kombiniert war (ViewRay). Es wurden jeweils 10 Fraktionen mit einer Einzeldosis von 3,85 Gy und der resultierenden Gesamtdosis von 38,5 Gy appliziert. Zur Reduktion der Hautdosis wurden pro Patientin 2 unterschiedliche Therapiepläne für jeweils 5 Fraktionen mit unterschiedlichen Tragarmwinkeln eingesetzt. Zur Abschirmung des unerwünschten Elektronenstrahls außerhalb der Strahlenfelder wurde ein 1 cm starker Bolus vor dem Kiefer sowie vor der ipsilateralen Schulter und dem Arm der Patientin positioniert. Vor dem Bolusmaterial (Richtung Brust) und hinter dem Bolusmaterial (Richtung Kopf) wurden EBT3-Radiochromröntgenfilme befestigt, um die ein- und austretende Dosis zu messen. Die Abhängigkeiten der Messwerte von der Lokalisation des bestrahlten Tumors, der Bestrahlungszeit und der Tumorgröße wurden untersucht.

Ergebnisse

Für jeweils eine Strahlenfraktion wurden auf den Röntgenfilmen vor dem Bolusmaterial (Richtung Brust) die von den einzelnen Isodosen eingeschlossenen Flächen vermessen: die 14 %-Isodosis (0,54 Gy) umschloss im Mittel eine Fläche von 3,6 cm2, die 12 %-Isodosis (0,46 Gy) im Mittel 10,4 cm2 und die 10 %-Isodosis (0,39 Gy) im Mittel 21,4 cm2. Die Röntgenfilme hinter dem Bolusmaterial zeigten hingegen keine signifikante Dosisbelastung an. Statistisch signifikante, aber nur geringe Korrelationen wurden zwischen der Dosisbelastung und den Behandlungszeiten beobachtet.

Schlussfolgerung

Werden Brustkrebspatientinnen mit einem MRT-geführten Tri-Kobalt-Bestrahlungsgerät behandelt, kann durch eine Wechselwirkung von Strahlenquelle und Magnetresonanztomograph ein Elektronenstrahl außerhalb der Therapiefelder unerwünscht entstehen. Dieser muss zur Schonung der Haut berücksichtigt und abgeschirmt werden.

Schlüsselwörter

Magnetfeld Magnetresonanztomographie-geführte Strahlentherapie Strahlenbelastung außerhalb der Therapiefelder Strahlenschutz Risikoorgane 

Notes

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1C1A1A01054192).

Compliance with ethical guidelines

Conflict of interest

J.M. Park, K.H. Shin, J.-i. Kim, S.-Y. Park, S.H. Jeon, N. Choi, J.H. Kim and H.-G. Wu declare that they have no competing interests.

Ethical standards

This study was approved by the institutional review board of Seoul National University College of Medicine/Seoul National University Hospital (IRB No. 1610-083-799).

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Copyright information

© Springer-Verlag GmbH Deutschland 2017

Authors and Affiliations

  • Jong Min Park
    • 1
    • 2
    • 3
    • 4
  • Kyung Hwan Shin
    • 1
    • 2
    • 3
    • 5
    Email author
  • Jung-in Kim
    • 1
    • 2
    • 3
  • So-Yeon Park
    • 1
    • 2
    • 3
  • Seung Hyuck Jeon
    • 1
  • Noorie Choi
    • 5
  • Jin Ho Kim
    • 1
    • 2
    • 3
  • Hong-Gyun Wu
    • 1
    • 2
    • 3
    • 5
  1. 1.Department of Radiation OncologySeoul National University HospitalSeoulKorea (Republic of)
  2. 2.Biomedical Research InstituteSeoul National University HospitalSeoulKorea (Republic of)
  3. 3.Institute of Radiation MedicineSeoul National University Medical Research CenterSeoulKorea (Republic of)
  4. 4.Robotics Research Laboratory for Extreme EnvironmentsAdvanced Institutes of Convergence TechnologySuwonKorea (Republic of)
  5. 5.Department of Radiation OncologySeoul National University College of MedicineSeoulKorea (Republic of)

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